The overall objective of the total project is to elucidate mechanisms involved in induction and maintenance of allografts in the rat. We have reported that survival of Lewis/BN allografts in Lewis recipients can be enhanced by treating the recipient with Lewis anti-BN alloimmune serum or Lewis/BN spleen cells or both. A spectrum of enhancement can be achieved by varying the dose, timing, and quantity of these biological reagents. However, the optimal regimen appears to be intravenous administration of 5 x 10 to the 7th power LBN (donor strain) spleen cells 11 days before transplantation and one ml. of hyperimmune Lewis anti-BN serum 10 days before transplantation. This regimen stimulates peak titers of an anti-idiotypic response against anti-BN receptors in the recipient by the day of transplantation. During the past two years most of our effort has been to generate mouse-rat monoclonal hybridoma antibodies of Lewis anti-BN specificity in an attempt to define more clearly the requirements of allograft enhancement. We have generated and tested more than 20 monoclonal mouse-Lewis hybridoma anti-BN supernatants. All are directed against Class I histocompatibility antigens. Four of them led to allograft enhancement when combined with donor strain LBN spleen cells in the -11 day, -10 day regimen. None of the supernatants enhanced graft survival when given alone. We propose to continue attempts to generate a mouse-Lewis rat hybridoma with specificity against Class I BN histocompatibility antigens. We will also combine various effective supernatants in an attempt to achieve enhancement without the need for concomitant donor strain spleen cells.